In the past, a control system of an internal combustion engine which is provided with an air-fuel ratio sensor at an exhaust passage of the internal combustion engine and controls the amount of fuel fed to the internal combustion engine based on the output of this air-fuel ratio sensor, has been widely known (for example, see PLTs 1 to 4).
In such a control system, an exhaust purification catalyst which is provided in the exhaust passage and has an oxygen storage ability is used. An exhaust purification catalyst which has an oxygen storage ability can remove the unburned gas (HC, CO, etc.), NOx, etc., in the exhaust gas flowing into the exhaust purification catalyst, when the oxygen storage amount is a suitable amount between an upper limit storage amount and a lower limit storage amount. That is, if exhaust gas of an air-fuel ratio at a rich side from the stoichiometric air-fuel ratio (below, also called a “rich air-fuel ratio”) flows into the exhaust purification catalyst, the unburned gas in the exhaust gas is oxidized and purified by the oxygen stored in the exhaust purification catalyst. Conversely, if exhaust gas of an air-fuel ratio at a lean side from the stoichiometric air-fuel ratio (below, also called a “lean air-fuel ratio”) flows into the exhaust purification catalyst, the oxygen in the exhaust gas is stored in the exhaust purification catalyst. Due to this, the surface of the exhaust purification catalyst becomes an oxygen deficient state. Therefore, the NOx in the exhaust gas is reduced and purified. As a result, the exhaust purification catalyst can purify the exhaust gas regardless of the air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst so long as the oxygen storage amount is a suitable amount.
Therefore, to maintain the oxygen storage amount in the exhaust purification catalyst at a suitable amount, the above control system is provided with an air-fuel ratio sensor at the upstream side of the exhaust purification catalyst in the direction of flow of exhaust and is provided with an oxygen sensor at the downstream side in the direction of flow of exhaust. By using these sensors, the control system performs feedback control based on the output of the upstream side air-fuel ratio sensor so that the output of this air-fuel ratio sensor becomes a target value which corresponds to a target air-fuel ratio. In addition, a target value of the upstream side air-fuel ratio sensor is corrected based on the output of the downstream side oxygen sensor. Note that, in the following explanation, the upstream side in the direction of flow of exhaust will sometimes simply be referred to as the “upstream side”, and the downstream side in the direction of flow of exhaust will sometimes simply be referred to as the “downstream side”.
For example, in the control system described in PLT 1, when the output voltage of the downstream side oxygen sensor is a high side threshold value or more and thus the state of the exhaust purification catalyst is an oxygen deficient state, the target air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst is set to a lean air-fuel ratio. Conversely, when the output voltage of the downstream side oxygen sensor is the low side threshold value or less and thus the state of the exhaust purification catalyst is an oxygen excess state, the target air-fuel ratio is set to the rich air-fuel ratio. According to PLT 1, due to this, when in the oxygen deficient state or oxygen excess state, it is considered that the state of the exhaust purification catalyst can be quickly returned to an intermediate state between these two states (that is, a state where the exhaust purification catalyst stores a suitable amount of oxygen).
In addition, in the above control system, if the output voltage of the downstream side oxygen sensor is between the high side threshold value and the low side threshold value, when the output voltage of the oxygen sensor tends to increase, the target air-fuel ratio is set to the lean air-fuel ratio. Conversely, when the output voltage of the oxygen sensor tends to decrease, the target air-fuel ratio is set to the rich air-fuel ratio. According to PLT 1, due to this, it is considered possible to prevent in advance the state of the exhaust purification catalyst from becoming an oxygen deficient state or oxygen excess state.
Further, in the control system described in PLT 2, the oxygen storage amount of the exhaust purification catalyst is calculated based on the outputs of the air flow meter and upstream side air-fuel ratio sensor of the exhaust purification catalyst. Moreover, when the calculated oxygen storage amount is larger than the target oxygen storage amount, the target air-fuel ratio of the exhaust gas flowing into the exhaust purification catalyst is set to the rich air-fuel ratio, while when the calculated oxygen storage amount is smaller than the target oxygen storage amount, the target air-fuel ratio is set to the lean air-fuel ratio. According to PLT 2, due to this, it is considered possible to constantly maintain the oxygen storage amount of the exhaust purification catalyst at the target oxygen storage amount.